The present invention relates to a vibration damper having a rebound, in particular having a rebound spring.
Such a vibration damper is described, for example, in German Published Patent Application No. 196 26 143 and has, on an axially movable piston rod, a piston which separates two working chambers in a cylinder. Throttle orifices are provided for connecting the two working chambers, which orifices can be formed in the piston, for example. The working chamber penetrated by the piston rod is closed off at an axial end with a lid which contains a piston rod guide. In the interior of this working chamber a rebound is arranged which is embodied as a rebound spring arranged concentrically with respect to the piston rod. The piston rod can perform stroke movements relative to the cylinder, namely out-strokes and in-strokes. If the vibration damper is mounted in a vehicle, it is supported on the bodywork of the vehicle and on an assigned vehicle wheel. The out-stroke of the piston rod corresponds to a rebound movement of the associated wheel, while the piston rod is retracted when the wheel experiences compression. As a result of the selected arrangement of the rebound spring in the vibration damper, the out-stroke of the piston rod includes an idle-stroke which starts at a minimum out-stroke and in which the rebound does not counteract the piston movement, a working-stroke which ends at a maximum out-stroke and in which the rebound counteracts the piston rod movement and a point of application at which the idle-stroke ends and the working-stroke begins.
In the conventional vibration damper, the rebound spring is secured at one of its axial ends to the piston and has a supporting ring at its other axial end. During the idle-stroke, the supporting ring is spaced axially apart from the lid of the cylinder. When the point of application is reached, the supporting ring comes to bear against the lid. In the working-stroke range, the rebound spring is supported by its axial ends on the lid and on the piston, as a result of which the rebound spring is stressed and counteracts the extension movement of the piston rod.
British Published Patent Application No. 812 528 also discloses a vibration damper with a rebound spring, the rebound spring being secured by one of its axial ends to the lid and by its other end to a stop piston. This stop piston is mounted so as to be axially adjustable in the cylinder and separates the working chamber penetrated by the piston rod into two component chambers which communicate with one another via an annular orifice formed between the stop piston and the piston rod. A ring which comes to bear against the stop piston at the point of application is formed on the piston of the vibration damper on a side facing the stop piston, the ring sealing the annular opening between the piston rod and stop piston and more or less throttling an exchange of fluid between the component chambers. Thus, the stop piston is supported on the lid not only by the rebound spring but also additionally by the hydraulic volume enclosed between the lid and the stop piston. This measure hydraulically amplifies the mechanical effect of the rebound spring during the working-stroke.
German Published Patent Application No. 28 06 540 discloses a further vibration damper whose rebound spring is supported between a stop piston, arranged in the cylinder, and the lid. A ring which is secured to the piston rod comes to bear against the stop piston at the point of application, as a result of which the stop piston is also moved, during which process the rebound spring is stressed. In an initial region of the working-stroke, the component chambers, separated by the stop piston, of the working chamber penetrated by the piston rod communicate with one another via a bypass. This bypass is embodied so that its passage area changes as a function of the stroke, the passage area being at a maximum at the point of application and decreasing as the out-stroke progresses. Thus, the effect of the rebound spring is increasingly supported hydraulically during the initial region of the working-stroke. The maximum degree of hydraulic support of the effect of the rebound spring occurs when the bypass is closed off.
U.S. Pat. No. 3,007,550 discloses a vibration damper having a rebound spring secured to the cylinder lid and to the piston. Accordingly, the idle travel of the piston rod has the value zero, with the result that the working-stroke occurs over the entire range of the out-stroke and the point of application corresponds to the minimum out-stroke. A secondary cylinder in which a secondary piston is mounted in an axially adjustable fashion is formed in the piston. A throttle gap is provided between the secondary piston and the piston rod and forms part of a throttled flow path through the piston and allows the two working chambers separated by the piston in the cylinder to communicate with one another.
The secondary piston can be adjusted in the secondary cylinder between an open position in which the throttle gap can be flowed through and a closed position in which the throttle gap is shut off. The secondary piston is biased into its open position with a spring arrangement. During an out-stroke, the throttle effect in the throttle gap generates, in the throttle gap, a driving force on the secondary piston which attempts to drive the secondary piston into its closed position. During out-strokes with relatively small stroke speeds, the bias force of the spring arrangement is greater than the throttle effect in the throttle gap, with the result that the secondary piston remains in its open position. However, starting from a certain stroke speed, the throttle effect in the throttle gap exceeds the bias force of the spring arrangement, as a result of which the secondary piston is moved into its closed position and interrupts a further exchange of fluid between the working chambers. The effect of the rebound spring is then supported by the hydraulic volume enclosed between the lid and piston.
In vehicles which are equipped with a ride control device, there is the problem that the rebound of the vibration damper can be configured only for a single ride level in terms of the stroke-dependent effect of the rebound. If, for example, there is an optimum ratio between the spring stiffness, idle-stroke and working-stroke for a normal level, this ratio inevitably changes when a ride level adjustment is performed. For example, the increase in the ride level leads to an out-stroke being set, with the result that the idle-stroke is reduced or becomes a zero. Likewise, the rebound spring may already be biased when this out-stroke occurs.
Problems of this type occur in particular in off-road vehicles if they can be adjusted at least between an on-road level and an off-road level by a ride level control device. A rebound spring which is configured for the on-road level and which has a relatively small idle-stroke for the sake of stable travel dynamics of the vehicle, is generally already biased at the off-road level, as a result of which the off-road capability is affected in terms of the overall spring travel values, the torsion capability of the wheels with respect to one another and in terms of the contact force between the wheel and underlying surface, and is thus adversely affected in terms of the traction of the vehicle. If these disadvantages are eliminated by configuring the rebound spring for the off-road level, an excessively large idle-stroke is produced at the on-road level, as a result of which the dynamic travel stability of the vehicle is significantly reduced.
It is an object of the present invention to provide a vibration damper in which the effect of the rebound is improved at different ride levels.
The above and other beneficial objects of the present invention are achieved by providing a vibration damper as described herein.
The present invention is based on the principle of configuring a vibration damper so that the point of application of the rebound may be set or adjusted. According to this measure it is possible to set an optimum ratio between the idle-stroke and working-stroke for any desired ride level. In particular, in the case of an off-road vehicle, it is therefore possible to set a point of application for an off-road level which is optimized in terms of the off-road capability, and to set a point of application for an on-road level which is optimized in terms of the dynamic travel stability.
In order to be able to set the ratio between the idle-stroke and working-stroke, the vibration damper according to the present invention includes an arrangement configured to adjust the point of application which may be of any desired configuration.
According to one example embodiment of the present invention, the arrangement configured to adjust the point of application includes an auxiliary piston which is arranged in the cylinder and which is mounted so as to be axially adjustable relative to the piston rod and relative to the cylinder, and by which the rebound is supported directly or indirectly on the lid or on the piston at least during the working-stroke, it being possible to adjust and/or lock the relative position between the auxiliary piston and piston rod or between the auxiliary piston and cylinder. Because the rebound is supported on the auxiliary piston at least during the working-stroke, it is possible to adjust the point of application of the rebound by the adjustment and/or locking of the relative position of the auxiliary piston with respect to the piston rod or with respect to the cylinder. This is because the working-stroke range for the rebound supported on the auxiliary piston starts at the latest when the auxiliary piston is locked relative to the cylinder or relative to the piston rod.
The auxiliary piston may be coupled, for example, to an actuator drive which positions and locks the auxiliary piston either relative to the piston rod, in which case the auxiliary piston is then adjustable relative to the cylinder, or positions and locks the auxiliary piston relative to the cylinder, in which case the auxiliary piston can then be adjusted relative to the piston rod. This actuator drive may include, for example, a spindle drive which interacts with the auxiliary piston.
However, in another example embodiment of the present invention, the auxiliary piston axially separates, in the cylinder in the working chamber penetrated by the piston rod, a first auxiliary chamber from a second auxiliary chamber, a volume adjusting arrangement being provided with which it is possible to adjust the hydraulic volume of that auxiliary chamber in which the auxiliary piston is supported on the lid or on the piston by the hydraulic volume set in the working-stroke range. In this example embodiment, the adjusting and locking of the auxiliary piston, and thus adjusting the point of application of the rebound, are achieved hydraulically. This is of particular advantage because a hydraulic arrangement is available in the cylinder of the vibration damper, with the result that the volume adjusting arrangement can be implemented, for example, with a simple valve arrangement. In addition, such a hydraulic solution has only a low degree of wear, resulting in a high service life for the vibration damper.
It should be appreciated that the features which are mentioned above and are to be explained below can be used not only in the respectively stated combination but also in other combinations or solely without departing from the scope of the present invention.